1. Field of the Invention
The present invention relates to fluid pressure to electrical signal transducers. More specifically, the present invention relates to transducers which employ differential pressure sensors to produce an electrical signal dependent upon a difference between two applied fluid pressures and to a pressure overload protection for protecting the pressure sensor from excessive fluid pressures.
2. Description of the Prior Art
Differential pressure transducers, or transmitters, using a sensor which have only a small displacement over their operating range are well-known in the art as shown in the U.S. Pat. No. 3,712,143 of Weaver et al. In such a transmitter, the sensor is typically a strain-sensitive semiconductor wafer with means for applying pressurized fluids to respective sides of the wafer to produce a physical displacement of the wafer in response to a differential pressure of the fluids. Such differential pressure transmitters include a pair of barrier diaphragms with a fluid chamber on one side of one barrier diaphragm being pressurized with an input fluid having a first pressure and a similar fluid chamber on one side of the other barrier diaphragm being pressurized with a second input fluid having a second pressure. A respective interior chamber on the other side of each of the barrier diaphragms contains a respective substantially incompressible fill fluid. These fill fluids communicate through fluid passages with opposite respective sides of the sensor wafer, and the wafer is displaced or deflected by an amount which is dependent upon the difference between the two pressures of the fill fluids. The sensor wafer, in turn, produce an electrical output signal which is dependent upon the displacement and, hence, is representative of a differential pressure of the fill fluids which, in turn, is dependent on the differential pressure of the input fluids. Such transmitters have usually incorporated pressure overload protection to prevent the sensor wafers from being excessively displaced by overload input differential pressures which could damage the sensor wafer. Such overload protection arrangements in the prior art have been based on providing equal or similar overload protection in both directions of displacement of the sensor wafer. However, it has been found that such sensor wafers are not symmetrical in their requirements for overload protection and, hence, require a predetermined ratio of overload protection, i.e., a different overload protection in one displacement direction than the other displacement direction. As a result, the prior art overload protection has been arranged to provide protection based on the lowest displacement strength capability of the sensor wafer which inherently provided substantial over-protection of the higher displacement capability and required stronger wafers to provide a displacement capability at the lowest level which was adequate to withstand a desired input pressure range before overload protection was necessary. Consequently, it would be desirable to provide overload protection which would be correlated, or matched, to the ratio of protection requirements of the sensor wafer rather than operating with an excessive mismatch between the overload protection and the sensor wafer to be protected.